I insist that my daughters where a helmet every ride.
It's hard to ask them to wear if I don't, So usually wear one when I ride with them. I wear one when riding young horse. But I will go with out when I ride with my cowboy friends and I'm on a very controllable horse.
That said, I've got stories of people who should have had helments on. I managed a CTR for many years and had one of our safety riders, bringing up the rear. His horse stumbled and went down, rolled over him and gave him a concussion. He spent a night in the local hospital.
Another girl was riding with us one day on a local ride. We were riding 3 a breast on a dirt rode at a good foxtrot. Her horse stumbled and planted it's nose in the rode and cartwheeled over.Slaming that girl face first into the dirt rode.
Two summers ago a death was reported in the paper of a rodeo star who had won many many barrel racing competitions, She was riding her trusted barrel horse and while crossing the road to pick up her mail, it slipped on the asphalt road and went down, breaking her neck. A hemet would not have saved her. I mention it to show that even very experienced riders on very experienced horses have accidents.
My daughters don't even ask not to wear it. They just automatically put them on.